STRUCTURE OF RUBY HILL. 21 



Rhyoiite eruption. — There has been another eruption of igneous rock in the 

 neighborhood of Ruby Hill, namely, the rhyoiite of Purple Mountain, 

 which is situated about a mile and a half east of the mines, but this erup- 

 tion could not have caused the deflection of the Ruby Hill formations to 

 the northwest, as it occurred subsequent to the original upheaval, although 

 it was no doubt intimately connected with the subsequent phenomena which 

 accompanied the deposition of the ore. 



Extent of the limestone on the surface. — The face of the quartzite mass which over- 

 lies the granite dips at an angle of about 40° northeasterlv under the lime- 

 stone of Ruby Hill. The exposure of this belt of limestone on the surface 

 extends over 4,000 feet from the northwest to the southeast. On the north- 

 west end it is covei'ed by the debris of Spring Valley, and on the southeast 

 is cut off by a fault which Mr. Hague has called the "Jackson fault" (Plate 

 I.). This brings it in contact with the Pogonip limestone lying to the east 

 of the Jackson hoisting works. Its width on the surface is from 800 to 

 2,000 feet. It shows few signs of stratification on the hill itself, and is 

 usually a compact highly crystalline limestone, gray in color and much 

 weathered. 



Extent of the shaie on the surface. — Beginning at the northwest, the shale first 

 makes its appearance at the Albion mine, follows round a promontory of 

 limestone, which extends nearly 1,000 feet to the north, and narrows down 

 to a point in the lower part of the town of Ruby Hill before reaching the 

 fault to the east. The shape of this shale body is very irregular; it is widest 

 to the north of the Richmond hoisting works, where it attains a width 

 of 2,400 feet, and narrowest, except where it comes to a point, north of the 

 above-mentioned promontory. Its course is nearly east and west, and its 

 dip northerly. The angle of dip of its stratification varies greatly. It 

 sometimes reaches 45°, but is frequently as small as 25°. 



Dip of the three formations. — It may be as well to state here that nowhere on 

 Ruby Hill does the dip of the stratification of any of the three formations, 

 quartzite, limestone, and shale, correspond with the dip of thair planes of 

 contact. In other words, the strata of these three formations do not con- 

 form to each other in their present position in the mines, though no doubt 

 they were originally laid down conformably. This lack of parallelism is 



